1. Field of the Invention
The present invention relates to a flow control valve for maintaining a constant flow rate of fluid (liquid or gas).
2. Description of the Related Art
The inventors of the present invention have disclosed, in Japanese Patent No. 2671183, a flow control valve employing diaphragms to secure a constant flow rate at the discharge (secondary) side of the valve irrespective of pressure variations at the inlet (primary) side thereof.
FIGS. 10 and 11 show this disclosure. The flow control valve 100 has a body 110 and a valve mechanism 120. The body 110 defines a chamber 130, which has an inlet 111 for receiving fluid to control, a valve seat 113 for passing the fluid, and an outlet 112 for discharging the fluid. The valve mechanism 120 has a valve plug 121 for opening and closing the valve seat 113, a first diaphragm 122 arranged on the inlet side, and a second diaphragm 123 arranged on the outlet side. The first and second diaphragms 122 and 123 divide the chamber 130 into a first pressure chamber 131, a valve chamber 135, and a second pressure chamber 140. A first pressurizing unit 151 is arranged in the first pressure chamber 131 and a second pressurizing unit 152 is arranged in the second pressure chamber 140, to inwardly (toward the valve chamber 135) apply constant pressure to the first and second diaphragms 122 and 123. In this example, the first pressurizing unit 151 is a spring and the second pressurizing unit 152 is pressurized gas. The valve 100 also has an intake port 141 for receiving the pressurized gas and a discharge port 142 for discharging the same.
In the valve 100, a pressure change at the inlet 111 works as a back pressure (outward pressure) on the first and second diaphragms 122 and 123. The valve mechanism 120 moves to balance the back pressure with the inward pressure applied by the pressurizing units 151 and 152. This results in moving the valve plug 121 to change an aperture between the valve plug 121 and the valve seat 113, thereby controlling the flow rate of the fluid controlled by the valve 100.
The valve mechanism 120 with the integrated diaphragms 122 and 123 provides a great advantage of quickly responding to instantaneous or pulsating pressure changes in the fluid controlled by the valve 100. This kind of pressure changes cannot be coped with by conventional valves having mechanical or electrical means. Further advantages of the valve 100 are the simple mechanism and structure and easy maintenance of the valve.
The valve 100, however, has presented new problems due to the integrated first and second diaphragms 122 and 123.
When the outlet 112 is clogged or closed, the second diaphragm 123 receives a large back pressure to apply large load on the valve mechanism 120. This large load may damage a joint (fastened with threaded parts) between the first and second diaphragms 122 and 123. When the back pressure at the outlet 112 moves the valve plug 121 to close the valve seat 113, large friction occurs between them due to the back pressure acting on the second diaphragm 123 and the pressure applied by the first pressuring unit 151 on the first diaphragm 122. This may break the valve plug 121 and valve seat 113 to produce dust particles.
These problems may lead to a failure of the valve 100 and deteriorate the function, durability, and reliability thereof. The dust particles, if produced, are hazardous to ultrapure water or chemical fluid for which such flow control valves are widely used.